Traditionally, earthen boreholes for oil and gas production, fluid injection, etc., frequently referred to as “wells,” were drilled by rotating a drillstring from the drilling rig, by means of a rotary table and kelly. The drill bit on the lowermost end of the drillstring was in turn rotated, and with the addition of weight applied to the drill bit by drill collars and other components of the drillstring, drilling took place.
An alternative way of rotating the drill bit is by means of a downhole device, either a downhole motor such as a positive displacement motor (frequently called a Moineau motor), or a downhole turbine. For purposes of this patent application, the term “downhole motor” will be used to broadly encompass any means of generating drill bit rotation, which is positioned downhole in the drillstring. Generally, when a downhole motor is being used, the drillstring is not rotated, or rotated slowly to reduce drag on the drillstring. Downhole motors utilize drilling fluid (“mud,” or in some cases gas) circulation, down through the drill string and through the downhole motor, to generate rotation (as described further below).
Downhole motors are also used in settings other than conventional drilling, for example with coiled tubing, or workstrings used in well cleanout work and the like.
Downhole motors, while taking various forms, generally comprise an outer housing which is fixed (generally by a threaded connection) to the drillstring, and a rotatable mandrel positioned within the housing and extending from the lowermost end of the housing. It is the mandrel that is rotated by means of fluid circulation through the drillstring and through the downhole motor. The drill bit is connected to the lowermost end of the mandrel, which usually has a “bit box” connection thereon. The mandrel therefore is free to rotate with respect to the housing, yet is fixed longitudinally within the housing.
Forces between the housing and the mandrel are both radial (side-to-side) and axial or thrust loads (acting along the longitudinal axis of the downhole motor). Radial bearings are positioned within the housing, between the housing and the mandrel, to take up the radial loads.
Thrust loads may be further separated into (1) loads or forces tending to push the mandrel out of the housing; and (2) loads or forces tending to push the mandrel up into the housing, or said another way, which are transferred from the housing to the mandrel to force it downward, such as to impose weight on the bit during drilling. With regard to the first category of thrust load, thrust bearings are positioned within the housing to sustain loads tending to force the mandrel axially out the lower end of the housing; such loads are generated by fluid circulation with the bit off bottom (such fluid pressure tending to push the mandrel out of the housing), or by pulling on the drill string with the bit and/or mandrel stuck in the hole. These thrust bearings will be referred to as secondary thrust bearings.
With respect to the first category of thrust load, in order to transmit a load to the drill bit, drillstring weight is transferred first to the housing, and from the housing to the mandrel, and thence to the drill bit. This downward weight or force transfer between the housing and mandrel is done by one or more thrust bearings, which for purposes of this application will be called the primary thrust bearings. Known prior art primary thrust bearings in downhole motors have taken various forms, including ball bearing assemblies, etc., all share one common structural attribute, namely that the primary thrust bearing assembly is contained within the bore of the housing, and positioned between the inner diameter of the housing and the mandrel. This limits the size of the primary thrust bearings which may be used, which in turn results in higher unit force (pressure) loads on the thrust bearings. Higher unit force loads result in increased wear and failure of the primary thrust bearings.
Yet other disadvantages exist with prior art designs. In such designs, a space or gap exists between the lowermost end of the housing and any upwardly facing surface (i.e. a shoulder) of the mandrel. This space creates an area of the mandrel which is exposed to the wellbore and fluids therein. Cuttings from drilling operations can damage or sever the mandrel at this unprotected location. Also, in through tubing or coiled tubing operations, this space creates a ledge or shoulder which can cause a motor to become lodged or stuck in the wellbore. Yet another disadvantage is increased length of the tool, due to placement of the thrust bearings within the body of the housing.